The present invention relates to a storage assembly for wafers.
A storage assembly of this kind is generally known in the prior art. Due to the continually increasing miniaturization and the increasing number of magazines, in practice it is unrealistic to have such magazines filled by an operator. Moreover, the associated costs are too high. For this reason, it is proposed in the prior art to move such cassettes with the aid of a robot. These cassettes may optionally be filled with wafers which are subjected to some form of treatment. In some devices, a robot of this kind is situated in a chamber which is filled with a particular gas, so that there is no longer any possibility of positioning them manually.
Hitherto, automatic positioning has been carried out by manually instructing the movement of the robot and storing the associated position data, i.e. the position of the robot with respect to the magazine, which is designed as a carousel. In this case, it was necessary to repeat this process after any possible disruption to the mutual position of robot and magazine, i.e. to reinput the relative positions. The position-determining means in the magazine did at least allow the cassettes to be arranged at an accurately defined position in the magazine. This is important for further processing and for allowing the wafers to be removed from the magazine accurately. Such disruption to the mutual position of the magazine and the robot takes place at any rate during initial assembly. For if a storage assembly of this kind is set with respect to one another during production, after trial runs the structure will be dismantled and reassembled at the ultimate place of use. If displacements occur in drive motors and the like during operation, it will be necessary to partially dismantle the device, with the result that the mutual position is likewise altered and resetting is required. A change in height can occur as a result of the base plate becoming warped, play which occurs during use and bending under the weight of the cassette in question, if the said cassette is full.
In particular if the number of magazines is relatively great, a setting operation of this kind is time-consuming, while in many cases the operator cannot easily gain access to the various stations, owing to lack of space. Moreover, a particularly accurate control system is required and it is necessary for the various components of the robot and also the rotating part of the carousel to be designed with particularly low tolerance. This is because after setting errors can no longer be corrected.
The object of the present invention is to avoid the disadvantages described above and to provide a storage assembly in which it is no longer necessary to carry out the complicated instruction of the robot with regard to positioning the cassettes accurately and in which it is not necessary to reinput the data manually after a disruption to the mutual positions of the various components, and in which, moreover, slight tolerances between the various moving parts are acceptable.
This object is achieved with a storage assembly having a magazine which is provided with compartments arranged at different heights for receiving cassettes. Each compartment has a base plate provided with means for establishing a predetermined position of the cassettes. A device for placing cassettes in the compartments is height adjustable, positionable in a horizontal plane and provided with a head for gripping the cassettes. The head of the device includes sensor means for determining a vertical position with respect to the magazine as well as a horizontal position thereof. Reference means interact with the sensor means for determining the horizontal position and are arranged in each of the compartments of the magazine. As a result of using sensor means and interacting reference means, each time a cassette is put in position the robot rechecks the extent to which the position of robot and magazine with respect to one another is correct. Then, any required correction is carried out. This takes place completely automatically. As a result, slight tolerances which occur during operation will have no effect on the operation, since the storage assembly according to the invention is self-teaching. Moreover, the stop position of the carousel is no longer critical and deviations are corrected automatically.
Sensor means may be present for determining the vertical position. These may be simple scanning sensors. The associated control system may, for example, be designed in such a way that after the initial contact between the relevant sensor means for determining the height and, for example, the base plate of the relevant compartment of the magazine, the device still moves a fixed distance downwards, in order to reach the correct position involved.
To determine the position in the horizontal direction, it is possible to use simple contact means or contactless means which determine the edge of the carousel. However, these do not allow an xy-position to be determined. Consequently it is necessary to know the position of a fixed point on the base plate. It is thus possible to arrange an opening or some other reference means in the base plate at a fixed position with respect to the position-determining projections for the cassette. If the sensor means on the head of the robot detect these reference means and direct themselves towards them, the correct position for putting down or picking up the cassette will then be reached automatically by moving a fixed distance.
This detectability of the reference means can be increased by providing auxiliary reference means. These cover a larger area, and as soon as the sensor means of the head are situated within this field, they can search for a fixed position with respect to the auxiliary reference means and, starting from this position, can find the main reference means described above. On this basis, the position for picking up or putting down the cassette in question can again be found.
According to a preferred embodiment, the above-described reference means for determining the horizontal position comprise an opening, while the sensor means comprise light-sensitive means. Light-sensitive means of this kind can determine whether or not light is incident through an opening of this kind without further contact being required. This light may comprise any light which is known in the prior art. It may be generated using conventional optical means, but it is also possible to use a laser beam.
If main and auxiliary reference means are used, in the case in which these means are designed as openings, the opening of the auxiliary reference means will be considerably larger than that of the main reference means. One example of such a diameter is 5 and 2 mm, respectively. The accuracy of the head with respect to the carousel must in all cases be + or xe2x88x922.5 mm. On this assumption, the light beam emanating from the sensor means strikes a boundary edge of the auxiliary reference means. By then carrying out a return movement, or some other movement, another edge of the opening of the auxiliary reference means is struck. By carrying out these reciprocating movements a number of times, it is possible to find the centre of the relevant opening of the auxiliary reference means in question. Then, the movement can be continued until the main reference means, i.e. the opening with the smaller diameter, is found, and then the position with respect to the cassette to be removed or the position-determining projections for positioning the cassette is established.
The above-described robot may comprise any structure which is known in the prior art. However, it preferably comprises a swivelling arm structure with the head arranged at one end and with the other end being height adjustable.